The purpose of fixing biological specimens is to preserve the sample as close to its natural state as possible so that it may be examined later. Conventional fixation methods typically use physical means such as heat or freeze-drying, or chemical means, such as aldehydes (e.g., formaldehyde, paraformaldehyde, or glutaraldehyde) or alcohols (e.g., ethanol or methanol) to preserve a biological specimen for further preparation and examination. In heat fixation, a slide with a biological specimen is passed through a flame to adhere the specimen to the slide. Other methods of heat fixation include exposing the sample to microwaves (with or without the presence of chemical fixatives in the specimen). Samples may also be fixed with cold temperatures by freeze drying. In chemical fixation, the sample is immersed in the fixative that penetrates the specimen to preserve the sample. Many chemical fixatives, such as aldehydes, act to crosslink the proteins in the cells or tissues. Precipitating fixatives, such as alcohols, cause the proteins in the sample to precipitate or aggregate by reducing the solubility of the proteins or disrupting their tertiary structure.
These conventional methods suffer from a variety of limitations. Importantly, fixation using one or more of these current methods may disrupt the structure and composition of the cellular components, creating fixed samples that do not truly reflect the properties of the living cells or tissues. Conventional fixation methods often take several hours, meaning that the sample structure and/or properties may be altered while the fixation is occurring. Fixation may distort or alter the sample due to, for example, shrinkage upon dehydration of the sample or swelling/shrinkage upon placement of the specimen in a non-aqueous chemical fixative. Some components of the specimen (e.g., lipids) may also be lost during the fixation process. In addition, the conventional methods typically require that the entire sample contained in the sample container (e.g., slide, dish, etc.) be fixed at once. Furthermore, almost all fixation chemicals are extremely hazardous and special precautions must be taken to avoid accidental and/or chronic overexposure. The hazards associated with these chemicals range from inhalation and skin/eye contact hazards to severe flammability.